You are correct, theoretically...
True, you can upholster the interior for a good price from a custom car shop.
Remember, this is factory OEM we talking about here --additional engineering, R&D, marketing they will factor in.
Just look at options for cars --over $100 for a replacement key, $3000 for Nav. The racing stripes option for the Viper is $5k and the better interior package (GTS) is $20k more (of coarse with other gadgets).
The MSRP of the current base corvette is $50k, maybe not quite $80k, but $70k is not far-fetched.
The new Corvette will have a nicer interior, but not current 911 quality, probably 996 quality. For that value, they would always have to compromise.